Of great commercial importance in such a printer is use of substantially in-line pens of different colors. In this type of configuration, a black pen k (FIG. 1, in which each vertical rectangle represents a separate pen), cyan pen c, magenta pen m and yellow pen y are all substantially aligned with one another--relative to the printing-medium-advance axis.
We say "substantially" because in practice it is often most economical in terms of manufacturing tolerances to allow slight misalignments of pen bodies--but to provide a few extra nozzles on each pen. Then end-masking can be selected automatically through a suitable calibration procedure to more nearly align the swaths actually produced by the several pens.
With such masking, certain nozzles are always unused, at least until one or more of the pens on a carriage is replaced. Calibration must then be done anew.
Such medium-advance-axis calibration procedures are set forth, for example, in the Cobbs and Sievert patent documents mentioned earlier. Thus, ignoring the always-unused nozzles near the ends of the pens, our reference to "substantially in-line pens" may be considered equivalent to "in-line swaths" or "in-line operating nozzle arrays".
Substantially in-line configurations are important because they minimize the length (along the printing medium) of the print zone, and thereby minimize both the overall envelope of the printing machine and the pen-to-print-medium distance. Housing a longer print zone expands the overall machine envelope--which is critical to cost, weight, installation space, inventory and shipping.
With a long print zone, commonly encountered amounts of printing-medium disturbance (flapping, crumpling, curling etc.)--per distance along the zone--accumulate to require longer pen-to-medium distance. The latter distance is critical to image quality, as longer flight distances amplify tolerances in nozzle-pointing angles. Ink drops also spread and spatter more while flying further.
A particularly long print zone also greatly complicates the configuration, weight and strength requirements of a pen carriage. These factors in turn implicate the carriage support and guide system, and thereby the drive motor, power consumption, heat dissipation etc.
Furthermore with in-line pens the procedures for printing at top and bottom of a page are simpler and easier to manage, and more importantly do not require operating with any of the pens in a position extended over the edge of the paper.
Also very important in a printer is speed, or throughput, measured as for example the number of pages that can be printed in a given time. For swath-based printers in general, the fastest and perhaps the most natural-seeming way of printing would be to abut swaths--the top of one swath immediately below the bottom of the preceding swath.
Such operation, however, would be acceptable only for a fast draft or rough preview mode of operation--or for printing text, in which swath boundaries sometimes can be kept between lines of text. Otherwise, defects in the printing tend to appear.
Such imperfections are due to irregularity in printing-medium advance mechanisms--or in nozzles near the ends of a pen. These manifest themselves very conspicuously as either shallow unprinted spaces or shallow double-printed strips, where swaths fail to abut or very slightly overlap, respectively.
A popular way of camouflaging such medium-advance defects is the use of deliberately overlapped swaths. In such a case each swath is only partially filled, so that completion of each one-swath-height-tall portion of the image actually takes more than one pass of the pens over the printing medium.
To take very simple examples, in each pass the pens may print only every other pixel row, or preferably every other pixel in an alternating checkerboard pattern. If that is done, then completion of each swath requires on average two passes.
A common way to operate in such a two-pass overlapped printing mode is to stagger successive passes by half the height of the pen array. Thus a first partially filled swath 1 (FIG. 9, in which each narrow rectangle now represents all pens that are present) is half-overlapped by a second partially filled swath 2. This pattern repeats in each successive pair 1, 2 of passes.
The simplest way to do this is to print while the pens are executing a pass 1 in one scan direction, for instance from left to right ("&gt;"in FIG. 9)--and then to return the pens to the left side of the print medium in a so-called "retrace" pass (not illustrated) as quickly as feasible in preparation for the next printing pass 2.
The benefit of a deliberate-overlap procedure is that the worst possible result of a slight imperfection in print-medium advance is a shallow space in which (for the checkerboard case) one sequence of dots appears in a given pixel row--rather than the ideal two interleaved sequences. This kind of shallow space with fifty-percent density (representing a fifty-percent error), resulting from partially-filled swaths being slightly too far apart, is far less conspicuous than a shallow space of zero density (a hundred-percent error, resulting from full swaths being too far apart).
Conversely, if partial swaths are too close together a shallow strip with three sequences of dots, rather than two, appears in a given row. The corresponding 150-percent density is far less conspicuous than a shallow strip of 200-percent density (again, errors of fifty and a hundred percent respectively).
It has been long recognized, however, that even a deadheading retrace still takes roughly half as long as a productive printing pass, so the total cost in time for each partial swath is roughly 11/2 times the actual printing time. In other words, ignoring startup and slowdown effects (and also ignoring the inefficiency of printing partially filled swaths at fifty-percent density), the time efficiency of the process is only about two-thirds.
One well-known way to mitigate this inefficiency and thus improve throughput in a scanning printer is to print bidirectionally--in other words, print dots during movement of the scanning pens in both scan directions. After each pass 1 (FIG. 10) in the forward direction ("&gt;") for each pair, the complementary pass 2 is printed on retrace ("&lt;").
In such operation it is desirable to take care that dots formed on retrace are properly in register, along the scan axis, with those formed in forward operation. Techniques for automatically measuring and correcting such registration in the field are taught by the previously mentioned patent documents of Cobbs, Sievert, and Raskin.
In printing of text, and some finely detailed color illustrations (particularly illustrations that do not happen to include heavy usage of secondary colors), this technique is generally satisfactory, although a great deal of work has been done with far more-elaborate print modes. In some of these modes, focusing on the limited liquid absorbency of some printing media, the percentage of fill--and the advance distance as a fraction of nozzle-array height--are smaller fractions than half, and the number of passes to complete inking of a region is a greater multiple than two. These print modes are particularly useful to minimize bleed of the ink, blocking or offset onto adjacent sheets, and cockle and warping of the medium (particularly with plastic media).
In other such innovations, addressing the directionality underlying print-medium-advance defects (especially end-of-page paper shrinkage, in systems with heating to accelerate drying of the paper), particular patterns of pixels are selected for each pass, to minimize the conspicuousness of such defects. A survey of numerous compensating print-mode techniques appears in the abovementioned application of Cleveland et al.; and related end-of-page control techniques are set out in the Broder et al. application.
In particular, Broder teaches a special structure of advances near the end of a page. Such a structure includes a decreasing advance distance--and eventually a rotating print-mask sequence--in an end-of-page zone where paper position is not controlled as well as elsewhere on the page. Cleveland teaches special masking to deal with an associated arch-shaped paper shrinkage.
For images that include solid blocks of color, however, and also even for some finely detailed color illustrations, a different kind of defect appears--still with reference to bidirectional printing using in-line pens (FIG. 10). Whereas a unidirectional printing regime creates secondary colors by always overprinting primaries in the same order--e.g. always magenta (m, FIG. 9) and then cyan (c) to make blue (cumulatively coded as "mc" in the FIG. 9 tabulation)--in conventional two-pass bidirectional printing the deposition order is opposite as between the forward and retrace passes.
In other words, the forward pass may deposit first m and then c (cumulatively coded as mc in FIG. 10), but on retrace the order is first c and then m (cm in FIG. 10). This difference arises because of the way, physically, in which the overprinted primaries are deposited.
Conventionally, in both unidirectional and bidirectional two-pass printing, two primaries are printed on a common pixel by discharge from two pens during the same pass. As the first pen passes over the pixel from left to right it discharges an inkdrop--which may be partly absorbed into the printing medium.
A tiny fraction of a second later a trailing pen flies over the same pixel and discharges another inkdrop on top of the first. For example if the first pen which inks the pixel is the magenta pen m (FIG. 1), and the second is the cyan pen c, the deposition sequence is mc (in FIG. 9 both columns for all rows; but in FIG. 10 the right-hand column in row 3c, and left-hand column otherwise) --with the cyan drop on top of the magenta.
On retrace of the same pens, of course the pens remain in the same physical order but they are moving in the opposite direction. This time, therefore, the cyan pen c arrives first and the magenta pen m later: the deposition sequence now is cm (left-hand column in rows 3c and 3e of FIG. 10, right-hand column otherwise), and with the magenta drop on top.
Unfortunately the ink which is deposited second tends to dominate visually, particularly if the drop from the first pass has already partly wicked into the medium. Furthermore, the second-deposited ink in the second pass dominates the second-deposited ink in the first pass, even though the second pass nominally deposits ink on different pixels than the first pass.
Perhaps this latter effect is due to the second-deposited ink in the second pass partially running off the first deposited ink onto adjacent pixels, where it overcoats both already-partly-dried inkdrops from the first pass (and where, therefore, much less wicking into the medium is now possible). In any event the overall effect of this defect can be appreciated simply by noting which color is deposited last of all in a given swath.
This can be seen for instance by comparing the right-hand column of color-deposition codes appearing in FIG. 10: m is last in row 3b, c in row 3c, and m again in row 3d. Thus the dominant color alternates at each half swath --swaths 3b and 3d are blue with a slight magenta cast, and intermediate swath 3c is blue with a slight cyan cast.
These small shifts create a banding artifact that is perceptible and objectionable in areas that should be solid blocks or regions of uniform color. The problem is similar for other secondary colors, and also for halftoned colors in solid blocks.
As mentioned above, even some finely detailed color illustrations exhibit this same artifact. This effect is strongly dependent upon the nature of each illustration.
More specifically, the artifact is particularly likely to arise where usage of secondary colors is particularly heavy--sometimes even with black overprinted on a color--if the finely detailed character of the image is not sufficient to break up the resulting banding pattern. To consider just one simple example: if an image region has many fine details of different colors but they are nearly all secondaries, then all of those different secondary colors as they appear in adjacent swaths of similar subject matter will appear as two different sets of slightly shifted secondary colors.
Thus the problem which we address is this: in bidirectional color printing with in-line pens which essentially print all colors in every pass, how can we avoid color shifts caused by deposition order--or at least minimize their conspicuousness?
One previous attempt to resolve a related problem appears in U.S. Pat. No. 4,855,752 of Bergstedt. Bergstedt discusses unidirectional printing exclusively.
He is not concerned with bidirectional printing, and does not face the problem of color shift arising from discrepancies in deposition order. The problem he addresses is color shift arising from discrepancies in drying/wicking times. He avoids or minimizes the appearance of such shifts by introducing offsets between bands of different colors--bands that overlap, to produce overprinted regions.
Bergstedt thus provides differential positioning as between swaths of individual colors. He discusses five different methods for accomplishing such offsetting of color bands. Two of his methods entail physical offsetting of different color pens--or different color nozzle arrays within a common, multicolor pen.
Besides not using bidirectional printing, the Bergstedt system--using physical offsetting--is of course antithetical to the problem which we address. Again, we aim to avoid deposition-order color shifts with pens that are substantially in-line.
A third method, of Bergstedt's five, does use in-line pens--but Bergstedt masks certain of the nozzles, near top or bottom of some pens, to provide the effect of staggered colors. In other words, he has in-line pens but not in-line swaths, and not in-line nozzle arrays.
It must be emphasized that in this method, as in all of his others, Bergstedt provides offsets between colors --that is, not between different multicolor swaths but rather between operations of the individual color-nozzle arrays, respectively. The significance of this distinction will become clear shortly.
Bergstedt mentions, however, that all his offsetting techniques summarized to this point do enable overprinting of colors together in a common pass. If this ability is exploited, in Bergstedt's unidirectional printing regime, the result for any of his first three methods will be to print overlapped swaths of multiple colors that are closely analogous to those created in the conventional two-pass printing mode (FIG. 8) we have described above.
It is important to recognize what would happen should Bergstedt attempt to operate his system bidirectionally. In any pixel rows where colors are overprinted together in a single pass, the deposition-order color shift which we have described would materialize. Thus the physical offsetting which Bergstedt describes is powerless to prevent color shifts that are caused by deposition order variation, in bidirectional printing.
In his fourth and fifth methods, Bergstedt uses inline pens but introduces his intercolor offsets through differential movements of the printing-medium advance mechanism, with respect to each color. In other words, he periodically backs up the print medium preparatory to printing of a certain primary color or colors.
Then he operates the advance mechanism forward again in preparation for printing of a certain other primary or primaries. If preferred he equivalently holds the printing medium stationary but moves the entire transverse-scanning mechanism backward along the medium-advance axis, prints a certain primary, and then moves the scan mechanism forward along the same axis and prints others.
Of course this method employs movements of, or relative to, the entire carriage and pen assemblage considered as a unit. Nevertheless, once again the point of these cyclical reversals is to provide relative positioning differences as between different colors.
Thus as Bergstedt emphasizes "these methods require that one pass of the printhead across the medium be made for each primary color deposited." With a separate pass for each color, whether laid down unidirectionally or bidirectionally, Bergstedt can provide much more consistent drying times.
He can thereby achieve excellent uniformity of secondary and compound colors--but at the cost of very low throughput. This cyclical-reversal strategy therefore is very much contrary to our present interest in producing uniform color as part of bidirectional printing for high throughput.
For best results Bergstedt advocates printing each color in an entirely separate pass--offset by the full common height of the nozzle arrays for the different colors (FIG. 10). In this favored mode, his pens never overprint two primaries in the same pass, since by definition of the geometry different primaries are laid down, on any given pixel row, in different passes.
Furthermore necessarily the cyan pen c (FIG. 10), magenta pen m and yellow pen y always reach each portion of the printing medium in that order, without variation. As a comparison with the in-line configuration (FIG. 1) makes clear, however, the resulting print zone is four times as long as the print zone of the present invention. Any such extension--even a relatively small fractional one--is very undesirable, as explained at the beginning of this section of the present document.
Bergstedt's discussion of drying-time color shift is relevant to the present invention in the sense that such a color shift forms a constraint on the use of embodiments of the present invention. This will be seen in the DETAILED DESCRIPTION section.
Another system has been used in a printer of the Epson Company, designated its model Stylus.TM. 720. Here inline color pens with nozzle spacing of 31/2 nozzle/mm (90 nozzle/inch), along the print-medium-advance axis, are used to create a pixel-grid spacing of about 14 or 28 pixel/mm (360 or 720 pixel/inch).
This is done by advancing the medium through very small increments equal to the pixel spacing, e.g., 1/14 mm (1/360 inch), between bidirectional passes 1-4 (FIG. 11). In other words, the system makes three consecutive advances (too small to be visible in the diagram) each one-quarter of the nozzle spacing.
Depending on the height of the nozzle array, each of these advances may, representatively, amount to only some one to three percent of the full nozzle-array height. These are followed by a much larger advance, e.g. ninety-seven to ninety-nine percent of the full height, to complete a full array-height excursion.
The overall pattern may be regarded as essentially four interleaved single-pass swaths 1-4 (FIG. 11) per cycle. It is essential to note that the pixel-row spacing (roughly 0.3 mm) produced within each swath individually --in other words, the spacing of the nozzles along the print-medium advance axis--is very different from the overall pixel-row spacing (about 0.07 or 0.035 mm) of the image.
The primary print order is the same from swath to swath, though it varies from row to row within each swath. While resolving deposition-order defects, this system is subject to the same problems of medium-advance defects as the abutted, nonoverlapping swaths discussed earlier.
Conclusion--Thus deposition-order color artifacts, and operating constraints imposed by their related drying-time artifacts, have continued to impede achievement of uniformly excellent inkjet printing at high throughput. Thus important aspects of the technology used in the field of the invention remain amenable to useful refinement.